On Fri, 19 Mar 2021 15:42:08 -0700  Mike Kravetz wrote:

+
+	if (!can_sleep && free_page_may_sleep(h, page)) {
+		/*
+		 * Send page freeing to workqueue
+		 *
+		 * Only call schedule_work() if hpage_freelist is previously
+		 * empty. Otherwise, schedule_work() had been called but the
+		 * workfn hasn't retrieved the list yet.
+		 */
+		if (llist_add((struct llist_node *)&page->mapping,
+					&hpage_freelist))
+			schedule_work(&free_hpage_work);
+		return;
+	}

Queue work on system_unbound_wq instead of system_wq because of blocking work.